1. Field of the Invention
The present invention relates to novel retinaldehyde binding proteins and the DNA encoding them, as well as to conjugate of such binding proteins to retinaldehyde, expression vector and host cells containing such DNA and antibodies to such binding protein.
2. Description of the Background Art
The visual pigments are the light-absorbing proteins in the retina. They have been studied for over 100 years as the centerpiece in the basic process of visual sensory transduction. Each individual pigment consists of a different seven-transmembrane-domain protein, called an opsin, that is bound to a chromophore, 11-cis-retinal (also known as 11-cis-retinaldehyde), via a Schiff base covalent bond. The 11-cis isomer is the chromophore of the majority of naturally occurring opsins. Exposure of opsin-bound retinal to light initiates the cis-trans isomerization and resultant dissociation of retinal from the apoprotein. This catalyzes an enzyme cascade which leads to visual excitation. Humans have four visual pigments, rhodopsin and the red, green, and blue cone pigments, all of which contain 11-cis-retinal as the native chromophore. Each human opsin is found specifically in the retinal photoreceptors, and none are known to be expressed in extraretinal tissue or use all-trans-retinal as a stably bound chromophore. The terms retinal and retinaldehyde are used interchangeably in the art and herein.
The retinal pigment epithelium (RPE) is a specialized cell monolayer that lies adjacent to the photoreceptors that are essential to the visual process. The RPE forms a monolayer of highly differentiated cells that segregates the photoreceptors from the choroidal capillaries. It constitutes part of the blood-retina barrier by regulating the transport of ions, nutrients, and macromolecules between the choroidal blood supply and photoreceptors. It is active in the uptake and storage of retinoids and in the secretion of basement membrane components. The polarized epithelial cells of the RPE are responsible for maintaining the photoreceptors by continually removing the shed fragments of outer segments through phagocytosis. The RPE performs this demanding phagocytic activity to enable each photoreceptor cell to renew its outer segment about every ten days.
The RPE has an important role in the formation of 11-cis-retinal and regeneration of the visual pigments. A lecithin retinol acyl transferase and an isomerohydrolase participate in the endothermic reaction of converting all-trans- to 11-cis-retinoids at the alcohol, rather than aldehyde, oxidation state. The substrate for the isomerohydrolase is an all-trans-retinyl ester, which is converted directly into 11-cis-retinol. The invertebrate visual cycle uses another method to regenerate active visual pigments. In molluscan eyes, there are a pair of photopigment systems, one containing rhodopsin and the other containing retinochrome. Photopigments are regenerated in the rhodopsin-retinochrome conjugate system by exchange of retinal chromophores. Retinochrome binds and photoisomerizes all-trans-retinal to 11-cis-retinal which is returned to the rhodopsin system by the retinal-binding protein, RALBP.
It is believed that aberrations in the functions of the RPE may lead to degeneration of the photoreceptors and to other visual disorders, including age-related macular degeneration (AMD). The molecular basis of RPE atrophy is unknown, and only a few macromolecules are known to be related to diseases of the RPE and retina.